The motor is the most frequently used rotary tool so far. With the development and popularization of the frequency converter, more and more motors need to be used together with the frequency converter. We have encountered many problems, and these exposed problems are more and more worthy of our deep thinking and communication.

1. Can the motor soft starter save energy?

The energy saving effect of soft start is limited, but it can reduce the impact of start on the power grid, and can also achieve smooth start and protect the motor windings.

According to the theory of energy conservation, due to the addition of a relatively complex control circuit, soft start not only does not save energy, but also increases energy consumption, but it can reduce the starting current of the circuit and play a protective role.

2. How about the starting current and starting torque of the motor when the inverter is used?

The inverter is used to operate, and the frequency and voltage are increased correspondingly with the acceleration of the motor, and the starting current is limited to below 150% of the rated current (depending on the model, it is 125%~200%). When starting directly with a commercial frequency power supply, the starting current is 6~7 times higher, so mechanical and electrical shocks will occur. Using frequency converter drive can start smoothly (starting time becomes longer). The starting current is 1.2~1.5 times of the rated current, and the starting torque is 70%~120% of the rated torque; for the frequency converter with automatic torque enhancement function, the starting torque is above 100%, and it can start with full load.

3. Is there any connection between the overload and short circuit of the motor?

There are two types of motor overload: 1. Mechanical load overload: it is the overload that drives the load beyond the rated value or the transmission system is stuck, which has nothing to do with the short circuit. 2. The load is normal: the motor current is overloaded, which may be a short circuit between the motor windings to the ground and between turns.

4. What is the application of frequency conversion speed regulation? What are the benefits?

It can be applied to rotating machinery with speed regulation requirements

Before the realization of frequency conversion speed regulation (it has been realized in theory, but the real realization is after the invention of power electronic devices), the traditional speed regulation adopts DC, and the disadvantages of DC speed regulation are:

1. The structure of the DC motor is complex and the maintenance cost is high

2. Due to the existence of the commutator, the power of the DC motor has little room for improvement

Therefore, the advantages of frequency conversion speed regulation are:

1. It can make the AC motor get the same excellent speed regulation performance as the DC speed regulation

2. The maintenance of AC squirrel cage asynchronous motor is simple and convenient

3. There is no limit to the commutator for AC motor power

5. Is it enough to use a 100KVA transformer to supply a total power of 100kW electrical appliances (maximum 37kw)? How much load can a 100KVA transformer carry?

P=capacity*power factor*80%=100*0.9*80%=72KW, generally 20% overload operation for 1 hour is allowed, so it is enough.

It mainly depends on whether the total current exceeds or not. The high-voltage current of a 100KVA transformer is 5.8A, and the low-voltage current is 150A. The temperature rise is equal to the actual temperature minus the ambient temperature. 6. How to measure the insulation resistance of the motor?

If it is a three-phase AC motor, measure the phase-to-phase and ground insulation resistance of the motor’s three-phase windings.

If it is a DC motor, measure the motor armature winding to ground, series excitation winding to ground, other excitation winding to ground, series excitation winding to other excitation winding. Select the corresponding megger according to the voltage level of the motor under test.

Measurement steps:

—Disconnect the power

—Discharge to ground

—If it is a three-phase AC motor, turn on the center point (if it is ok)

—If it is a DC motor, lift the brush.

—Use a megger to detect the phase-to-phase and ground-to-ground insulation resistance respectively

—Discharge to ground

— restore line

—Record insulation resistance and ambient temperature.

7. What is a brushless ringless starter?

The brushless and ringless starter is a starting device that overcomes the shortcomings of the wound asynchronous motor with slip rings, carbon brushes and complicated starting devices, while retaining the advantages of small starting current and large starting torque of the wound motor. . All JR , JZR , YR , YZR three-phase wound-rotor AC asynchronous motors originally started by resistance starters, reactors, frequency-sensitive rheostats, liquid rheostat starters, and soft starters (except those with variable speeds and cameras) “Brushless and ringless starter” can be selected for replacement.

8. How many ways are there for capacitor starting of the motor?

There are two starters:

1. Capacitor start (referring to the disconnection of the capacitor after the motor is started);

2. The capacitor starts and runs (the capacitor participates in the operation after starting).

9. Can the transformer be used as the load of the inverter?

In principle, it should be possible, but it is not practical in practice. Even if the inverter does not need a transformer to boost the voltage, there should be varieties that can be used in circuits above 380V. If you want a higher voltage, you can also directly use 220V or The 380V direct frequency conversion and then the voltage doubling method to obtain high voltage circuit can be used.

The frequency converter is mainly used for load driving (such as motor), and is rarely used for power frequency conversion, and the function of the frequency converter is far from limited to the frequency conversion itself, and there are many additional functions, such as various protections, etc. If the frequency converter is used It is not advisable from an economic point of view to obtain a variable frequency power supply. It is recommended to use other frequency conversion circuits.

10. Can the inverter be adjusted to 1Hz? How much HZ can be adjusted at the highest?

If the frequency converter is used on a general AC asynchronous motor, it is absolutely impossible when the frequency converter is adjusted to 1Hz and it is close to DC. The motor will run at the maximum current within the limit of the frequency converter, and the motor will heat up severely, which is very likely Burn out the motor.

If the operation exceeds 50Hz, it will increase the iron loss of the motor, which is also unfavorable to the motor. Generally, it is best not to exceed 60Hz, (it is allowed to exceed in a short period of time), otherwise it will also affect the service life of the motor.

11. What is the working principle of the frequency adjustment resistor of the inverter? Why can the frequency be changed by adjusting the resistor?

The frequency adjustment resistor of the frequency converter is used to proportionally divide the 10V reference voltage of the frequency converter, and then send it back to the main control board of the frequency converter. The main control board of the inverter then converts the voltage sent back by the resistor into analog-to-digital conversion to read the data, and then converts it into a proportional value of the rated frequency to output the current frequency. Therefore, adjusting the resistance value can adjust the frequency of the inverter.

12. Can the inverter decouple the motor current?

Can frequency conversion be decoupled? No.

However, as long as the output frequency f and synchronous speed n1 keep the slip rate in the stable area or the rated slip rate Se, it is equivalent to decoupling the motor current, because the rotor power factor is 1 at this time, and the rotor current is what we need to decouple The torque current to be controlled! The frequency converter is the speed control device of the asynchronous motor, and it is impossible to perform any so-called control beyond the mechanical characteristics of the asynchronous motor!

13. Why is the current large when the induction motor starts? And the current will become smaller after starting?

When the induction motor is in a stopped state, it looks like a transformer from an electromagnetic point of view. The stator winding connected to the power supply is equivalent to the primary coil of the transformer, and the closed-circuit rotor winding is equivalent to the secondary coil of the transformer short-circuited; the stator winding There is no electrical connection with the rotor winding, only a magnetic connection, and the magnetic flux forms a closed circuit through the stator, air gap, and rotor core.

At the moment of switching on, the rotor has not rotated due to inertia, and the rotating magnetic field cuts the rotor winding at the maximum cutting speed—the synchronous speed, so that the rotor winding induces the highest possible potential, so a large current flows in the rotor conductor. Current, this current produces magnetic energy that cancels the stator magnetic field, just as the secondary magnetic flux of the transformer needs to cancel the primary magnetic flux.

In order to maintain the original magnetic flux compatible with the power supply voltage, the stator automatically increases the current. Because the rotor current is very large at this time, the stator current also increases greatly, even as high as 4 to 7 times the rated current, which is the reason for the large starting current.

Why is the current small after starting: As the motor speed increases, the speed at which the stator magnetic field cuts the rotor conductor decreases, the induced potential in the rotor conductor decreases, and the current in the rotor conductor also decreases, so the stator current is used to offset the rotor current. The part of the current affected by the magnetic flux also decreases, so the stator current changes from large to small until it is normal.

14. What impact does the carrier frequency have on the inverter and the motor?

The carrier frequency has an influence on the output current of the inverter:

(1) The higher the operating frequency, the greater the duty cycle of the voltage wave and the smaller the current high-order harmonic component, that is, the higher the carrier frequency, the better the smoothness of the current waveform;

(2) The higher the carrier frequency, the smaller the allowable output current of the inverter;

(3) The higher the carrier frequency, the smaller the capacitive reactance of the wiring capacitance (because Xc=1/2πfC), and the larger the leakage current caused by the high-frequency pulse. The influence of carrier frequency on the motor:

The higher the carrier frequency, the smaller the vibration of the motor, the smaller the running noise, and the less heat generated by the motor. However, the higher the carrier frequency, the higher the frequency of the harmonic current, the more serious the skin effect of the motor stator, the greater the loss of the motor, and the smaller the output power.

15. Why can’t the inverter be used as a variable frequency power supply?

The entire circuit of the variable frequency power supply is composed of AC-DC-AC-filtering and other parts, so its output voltage and current waveforms are pure sine waves, which are very close to the ideal AC power supply. It can output the grid voltage and frequency of any country in the world.

The frequency converter is composed of AC-DC-AC (modulated wave) and other circuits, and the standard name of the frequency converter should be frequency converter. The waveform of its output voltage is a pulsed square wave, and there are many harmonic components. The voltage and frequency change proportionally at the same time, and cannot be adjusted separately, which does not meet the requirements of AC power supply. In principle, it cannot be used as a power supply, and is generally only used for speed regulation of three-phase asynchronous motors. 16. When using a frequency converter, why is the temperature rise of the motor higher than that at power frequency?

Because the output waveform of the inverter is not a sine wave, but a deformed wave, the motor current under the rated torque is about 10% more than that at the power frequency, so the temperature rise is slightly higher than that at the power frequency.

Another point: when the motor speed decreases, the speed of the motor cooling fan is not enough, and the temperature rise of the motor will be higher.

17. What does the protection level of the motor mean?

For example, an IP23 motor means that the motor can prevent the intrusion of solid objects larger than 12mm, prevent human fingers from touching internal parts, and prevent the intrusion of medium-sized (larger than 12mm in diameter) foreign objects. It can prevent the intrusion of sprayed water, or prevent the intrusion of water sprayed in a direction with an angle of less than 60 degrees from the vertical to cause damage.

IP (INTERNATIONAL PROTECTION) protection level system is drafted by IEC (INTERNATIONAL ELECTROTECHNICAL COMMISSION). Classify the motor according to its dustproof and moistureproof characteristics. The foreign objects referred to here, including tools, human fingers, etc., must not touch the live parts of the motor to avoid electric shock. The IP protection level is composed of two numbers. The first number indicates the level of the motor from dust and the intrusion of foreign objects, and the second number indicates the airtightness of the motor against moisture and water intrusion. The larger the word is, the protection level higher.

Reviewing editor: Tang Zihong

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